Carbon fiber reinforced composite materials are useful because of being excellent in strength, rigidity, electrical conductivity, and the like and are widely deployed in aircraft structural members, windmill blades, automobile outer panels, computer applications such as IC trays and casings (housings) of notebook computers, and the like, leading to yearly increase in demand.
Carbon fiber reinforced composite materials are in general heterogeneous materials formed by molding a prepreg having carbon fibers as reinforcing fibers and a matrix resin as essential components. Owing to this structure, there is a large difference between mechanical characteristics in the arrangement direction of the reinforcing fibers and mechanical characteristics in other directions. It is known, for example, that impact resistance represented by resistance to drop-weight impact is governed by interlayer delamination strength quantified by interlayer plate-end delamination strength or the like, and improvement in the strength of the reinforcing fibers alone does not lead to radical improvement. In particular, carbon fiber reinforced composite materials having a thermosetting resin as a matrix resin are of such a nature that they are liable to be broken by stress in directions other than the arrangement direction of the reinforcing fibers, reflecting low toughness of the matrix resin. In view of this, various techniques have been disclosed for the purpose of improving the mechanical characteristics of composite materials so that they can withstand the stress in directions other than the arrangement direction of the reinforcing fibers.
As one of among them, a prepreg is disclosed that includes a resin layer in which resin particles are dispersed in the surface area of the prepreg. For example, a technique is disclosed that provides a high-toughness carbon fiber reinforced composite material having favorable heat resistance using a prepreg that includes a resin layer in which particles formed of a thermoplastic resin such as nylon are dispersed in the surface area of the prepreg (see Patent Literature 1). Apart therefrom, a technique is disclosed that causes a carbon fiber reinforced composite material to exhibit high toughness through a combination of a matrix resin whose toughness is improved by the addition of a polysulfone oligomer and particles formed of a thermosetting resin (see Patent Literature 2). However, while these techniques provide carbon fiber reinforced composite materials with high levels of impact resistance, they produce a resin layer as an insulating layer in an interlayer. This structure causes a drawback that electrical conductivity in the thickness direction out of electrical conductivity as one of the features of carbon fiber reinforced composite materials remarkably decreases, and it is difficult to achieve both excellent impact resistance and electrical conductivity in carbon fiber reinforced composite materials.
As a method for improving interlayer electrical conductivity, there are a method in which metallic particles are formulated in a matrix resin of a carbon fiber reinforced composite material (see Patent Literature 3) and a method in which carbon particles are formulated in a matrix resin (see Patent Literature 4). However, these literatures make no reference to achieving both impact resistance and electrical conductivity at a high level.
For the purpose of improving the adhesiveness and bindability of carbon fibers, various sizing agents for carbon fibers have been disclosed. Examples of the disclosed sizing agents include a compound having a plurality of epoxy groups of an aliphatic type, an epoxy adduct of a polyalkylene glycol, diglycidyl ether of bisphenol A, a polyalkyleneoxide adduct of bisphenol A, and a polyalkyleneoxide adduct of bisphenol A with an epoxy group added. However, any sizing agent formed of one type of epoxy compound imparts insufficient adhesiveness and bindability to carbon fibers. A method using two or more types of epoxy compounds in combination according to required functions has been disclosed in recent years.
For example, a disclosed sizing agent includes two or more epoxy compounds each having a defined surface energy (see Patent Literatures 5 to 8). Patent Literature 5 discloses a combination of an aliphatic epoxy compound and an aromatic epoxy compound. Patent Literature 5 describes that a sizing agent present in the outer layer in a large amount has an effect of shielding another sizing agent present in the inner layer in a large amount from air, and this prevents the epoxy group form undergoing ring-opening by water in air. Patent Literature 5 also describes that the sizing agent preferably contains the aliphatic epoxy compound and the aromatic epoxy compound in a ratio of 10/90 to 40/60, and the aromatic epoxy compound is preferably contained in a larger amount.
Patent Literatures 7 and 8 disclose sizing agents containing two or more types of epoxy compounds having different surface energy. Patent Literatures 7 and 8, which have an object of improving adhesiveness with a matrix resin, do not specify the combined use of an aromatic epoxy compound and an aliphatic epoxy compound as a combination of two or more types of epoxy compounds and provide no general exemplification of aliphatic epoxy compounds selected from the viewpoint of adhesiveness.
Another disclosed sizing agent contains a bisphenol A epoxy compound and an aliphatic polyepoxy resin in a mass ratio of 50/50 to 90/10 (see Patent Literature 9). However, the sizing agent disclosed in Patent Literature 9 also contains the bisphenol A epoxy compound as an aromatic epoxy compound in a large amount.
A disclosed sizing agent specifying the combination of an aromatic epoxy compound and an aliphatic epoxy compound is a combination of a multifunctional aliphatic compound on the surface of carbon fiber bundles and an epoxy resin, a condensate of an alkylene oxide adduct with an unsaturated dibasic acid, and an alkylene oxide adduct of a phenol on the surface of the multifunctional aliphatic compound (see Patent Literature 10).
A disclosed combination of two or more epoxy compounds is a combination of an aliphatic epoxy compound and a bisphenol A epoxy compound as an aromatic epoxy compound. The aliphatic epoxy compound is a cyclic aliphatic epoxy compound and/or a long chain aliphatic epoxy compound (see Patent Literature 11).
A combination of epoxy compounds having different properties has also been disclosed. A disclosed combination contains two epoxy compounds that are liquid and solid at 25° C. (see Patent Literature 12). Furthermore, a combination of epoxy resins having different molecular weights and a combination of a monofunctional aliphatic epoxy compound and an epoxy resin have been developed (see Patent Literatures 13 and 14).
However, the sizing agents (for example, Patent Literatures 11 to 14) containing two or more components practically fail to achieve both the adhesion between carbon fibers and a matrix resin and the stability of a prepreg during long-term storage. The reason is considered as follows: The following three requirements are needed to be satisfied in order to simultaneously achieve the high adhesion and the suppression of the reduction in mechanical characteristics of a prepreg during long-term storage, but a conventional combination of any epoxy resins has failed to satisfy these requirements. Of the tree requirements, the first is that an epoxy component having high adhesion is present in the inner side (carbon fiber side) of a sizing layer, and the carbon fibers and the epoxy compound in the sizing interact strongly; the second is that the surface layer (matrix resin side) of the sizing layer has a function of suppressing the reaction between a matrix resin and the epoxy compound that is present in the inner layer and that has high adhesion to carbon fibers; and the third is that the surface layer (matrix resin side) of the sizing agent necessitates a chemical composition capable of strongly interacting with a matrix resin in order to improve the adhesion to the matrix resin.
For example, Patent Literature 5 discloses a sizing agent having an inclined structure for increasing the adhesion between carbon fibers and the sizing agent, but Patent Literature 5 and any other literatures (for example, Patent Literatures 6 to 9) have no idea that the sizing layer surface simultaneously suppresses the reaction between an epoxy compound having high adhesion to carbon fibers and a component in a matrix and achieves high adhesion to the matrix resin.
Patent Literature 10 discloses a sizing agent including an inner layer containing a multifunctional aliphatic compound and an outer layer containing an aromatic epoxy resin and an aromatic reaction product each having low reactivity. The sizing agent should prevent a prepreg stored for a long period of time from suffering change with time, but the surface layer of the sizing agent contains no multifunctional aliphatic compound having high adhesion, and this makes it difficult to achieve high adhesion to a matrix resin.